The manufacture of cigarette filters from a continuous, multifilament filter tow generally involves processing steps which include the opening up of the tow or separation of the individual filaments, the application of plasticizer or other additives to the opened up tow and the formation of a continuous filter rod from the treated filter tow. The uniformity and filtering characteristics of the resulting filter rod are largely determined by the effectiveness of these tow processing steps. Thus, there is a substantial amount of prior art which is directed to methods and apparatus for transforming filter tow into cigarette filters having predictable smoke filtration characteristics.
U.S. Pat. No. 2,966,198 discloses a method and apparatus for processing filter tow wherein unopened tow is passed through a cylindrical tube in which the tow is subjected to a turbulent air stream and a spraying treatment with an aqueous solution of a cellulose derivative. That method and apparatus, however, do not lead to a filter rod that has acceptable uniformity because the turbulent air zone provided is insufficient to open up the tow before the tow is sprayed with the aqueous treating solution. This is due in part to the fact that the turbulent air stream is introduced at a point immediately upstream of the spraying device. The result is an uneven distribution of treating solution on the advancing tow. Also, the apparatus design is such that substantial quantities of compressed air are required to maintain the turbulence in the turbulent air zone.
Other methods and apparatus for treating filter tow with an air stream in tow confining means having substantially circular cross-sectional configurations are disclosed in U.S. Pat. Nos. 3,099,594, 3,262,181, 3,282,768 and 3,297,506. In each of these patents a continuous, multifilament filter tow is passed through a jet tube containing orifice means where the tow is combined with an air stream at elevated pressures. The combined filter tow/air stream emerges from the confining walls of the tubular member into an exit member which allows the elevated pressures to dissipate rapidly thereby causing the filter tow to be bloomed. Of particular interest is an alternative embodiment shown in FIG. 5 of U.S. Pat. No. 3,282,768 wherein plasticizer is injected into the tow blooming apparatus via a capillary tube probe terminating at the orifice means positioned in the jet tube. Injection of plasticizer at this point, however, is completely unsatisfactory because the filter tow emerging from the orifice means has not yet assumed an expanded or "opened up" configuration due to the action of the air stream. This results in plasticizer being applied only to the tow fibers immediately surrounding the capillary tube probe and the air currents associated with the filter tow downstream from the orifice means are unable to promote uniform redistribution of the plasticizer throughout the tow. When capillary tube probes are positioned at the inner wall surface of the jet tube as shown in FIG. 5 of U.S. Pat. No. 3,282,768, there is a similar inability of the air stream to effect uniform redistribution of plasticizer throughout the filter tow. Unless the quantity of plasticizer flow to the capillary tube probes is carefully controlled and limited, there is a tendency for droplets of plasticizer to collect on the inner wall surface of the jet tube and to move under the influence of the air stream to the exit end of the jet tube where the plasticizer drips from the apparatus. The problems in achieving satisfactory operation of this alternative embodiment are apparently appreciated by the patentees in that the preferred apparatus embodiment disclosed in FIG. 6 of U.S. Pat. No. 3,282,768 employs wick type applicators for applying plasticizer to a flattened band of filter tow before the tow is introduced into the jet blooming device. In fact, the most widely used commercial methods for applying plasticizer to filter tow in the manufacture of cigarette filters currently involve the application of plasticizer to a flat band or web of the filter tow.